New Impact Rate Count Lays Nemesis Theory to Rest

Is the Earth more likely or less likely to be hit by an asteroid or comet now as compared to, say, 20 million years ago? Several studies have claimed to have found periodic variations, with the probability of giant impacts increasing and decreasing in a regular pattern. Now a new analysis by Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA), published in the Monthly Notes of the Royal Astronomical Society, shows those simple periodic patterns to be statistical artifacts. His results indicate either that the Earth is as likely to suffer a major impact now as it was in the past, or that there has been a slight increase impact rate events over the past 250 million years.

The results also lay to rest the idea of the existence of an as-yet undetected companion star to the Sun, dubbed “Nemesis.”
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth’s surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.

Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.

Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.

One proposed mechanism for these variations is the periodic motion of our Solar System relative to the main plane of the Milky Way Galaxy. This could lead to differences in the way that the minute gravitational influence of nearby stars tugs on the objects in the Oort cloud, a giant repository of comets that forms a shell around the outer Solar System, nearly a light-year away from the Sun, leading to episodes in which more comets than usual leave the Oort cloud to make their way into the inner Solar System – and, potentially, towards a collision with the Earth. A more spectacular proposal posits the existence of an as-yet undetected companion star to the Sun, dubbed “Nemesis”. Its highly elongated orbit, the reasoning goes, would periodically bring Nemesis closer to the Oort cloud, again triggering an increase in the number of comets setting course for Earth.

For MPIA’s Coryn-Bailer-Jones, these results are evidence not of undiscovered cosmic phenomena, but of subtle pitfalls of traditional (“frequentist”) statistical reasoning. Bailer-Jones: “There is a tendency for people to find patterns in nature that do not exist. Unfortunately, in certain situations traditional statistics plays to that particular weakness.”

That is why, for his analysis, Bailer-Jones chose an alternative way of evaluating probabilities (“Bayesian statistics”), which avoids many of the pitfalls that hamper the traditional analysis of impact crater data. He found that simple periodic variations can be confidently ruled out. Instead, there is a general trend: From about 250 million years ago to the present, the impact rate, as judged by the number of craters of different ages, increases steadily.

There are two possible explanations for this trend. Smaller craters erode more easily, and older craters have had more time to erode away. The trend could simply reflect the fact that larger, younger craters are easier for us to find than smaller, older ones. “If we look only at craters larger than 35 km and younger than 400 million years, which are less affected by erosion and infilling, we find no such trend,” Bailer-Jones explains.

On the other hand, at least part of the increasing impact rate could be real. In fact, there are analyses of impact craters on the Moon, where there are no natural geological processes leading to infilling and erosion of craters, that point towards just such a trend.

Whatever the reason for the trend, simple periodic variations such as those caused by Nemesis are laid to rest by Bailer-Jones’ results. “From the crater record there is no evidence for Nemesis. What remains is the intriguing question of whether or not impacts have become ever more frequent over the past 250 million years,” he concludes.

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8 Replies to “New Impact Rate Count Lays Nemesis Theory to Rest”

I have heard about and read about the infamous “Nemesis” for years. I have enjoyed it immensely. At the same time, however, I have read at least as many (in reality tremendously more) science, physics, astrophysics, and astronomy books. Reading science fiction novels is quite enjoyable, but reading science fact is much more to my liking. To many believers in Nemesis, no amount of real science will ever convince them that it doesn’t exist. I have been fascinated by all the dooms day talk about what will happen on Dec. 21, 2012. Some say that Nemesis will make its presence known. Others pair Nemesis with a mysterious “Planet X.” Oh what joy! Thanks for your article. I look forward to reading more on Dec. 22nd.

This wasn’t totally unexpected. The same problem happened with the fossil record, where seeming patterns of periodic increases in extinction rates were found. With better data and better statistical methods, these patterns went away. [“Dynamics of origination and extinction in the marine fossil record”, John Alroy, PNAS 2008.]

I am fairly sure similar problem adheres to crater counting, where erosion and spotty discovery happens on one side, and naive statistics can happen on the other. The most pressing issue could be that the early bombardment tail is inconclusive.

And even if there were a Last Heavy Bombardment impact peak that fits Nice planet formation and migration models, there are papers on impacts that forms a longer tail than in the consensus model. IIRC a tail to 3.2 Ga, when the consensus says to ~ 3.8 – 3.5 Ga.

For MPIA’s Coryn-Bailer-Jones, these results are evidence not of undiscovered cosmic phenomena, but of subtle pitfalls of traditional (“frequentist”) statistical reasoning. Bailer-Jones: “There is a tendency for people to find patterns in nature that do not exist. Unfortunately, in certain situations traditional statistics plays to that particular weakness.”

He would say that if he is a bayesian battling frequentists. He also notes that his approach uses models so avoids ad hoc statistics.

However, model approaches can be used in statistics, and bayesian methods can be ad hoc. Specifically, Alroy do not use models to test for lack of patterns. (He uses autocorrelation methods.)

Potato, potatoe.

Btw, a nitpick or proposed clarification:

Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago.

Not linked on an individual basis in general. What has been firmly proposed has been the above mentioned extinction pattern. It has been rejected by some later work.

Other suspected extinction event impacts have turned out to be products (even in cases suspected fabrications, I think) of too eager minds.

The proposed India K-Ph impact that was suggested as the causal factor has never been validated, and it has been criticized.

The proposed North America Younger Dryas cold period impact has been firmly excluded by various means as this example shows: “Whereas proponents of the theory have offered “carbonaceous spherules” and nanodiamonds, both of which they claimed were formed by intense heat as evidence of the impact, a new study concludes that those supposed clues are nothing more than fossilized balls of fungus, charcoal, and fecal pellets.” Climate science has also been critical, as well as a noted absence of widespread fires and other expected evidence.

I am not sure where you got this from. The original planet Vulcan was a putative planet inside the orbit of Mercury. This was conjectured to account for the perihelion advance of Mercury which could not be accounted for by the perturbations of the known planets. The solution to the problem was the general theory of relativity which did not conserve the argument of periapsis. This was the anomalous perihelion advance.

As a side note, the solution to this problem came about not by introducing more degrees of freedom, such as 6 orbital parameters of some new planet. It came about with a new theoretical framework which removed the invariance of the periapsis. We face a similar issue IMO with the foundations of physics.

The planet was deemed Vulcan because it was close to the sun. The Greek god Vulcan was the god of volcanoes and the smelter of metals. IOW Vulcan was the god of hot stuff. A putative planet out far enough that its orbital period is nearly 5000 years would be a cold place indeed.

There is no planet between Mercury and the Sun. I believe it was due to the early recognition of sunspots. The name Vulcan comes from Madam Blavatsky’s theosophical work. Too bad astronomers did not read her work or they could have figured out its orbit elements.

For example, from Blavatsky’s work and other data.

Vulcan’s Theoretical Period = 4969.0 years +/- 5.7 (one sigma) years.

But the period can also be figured out in four other ways. When one statistically combines these values, one gets.

This body never comes near the inner solar system, with perigee around 130 Au and aphelion around 448 AU.

But the comet swarms it generates do threaten Earth, so some of this data relies on ice core and tree ring data. Other is related to giant comet CR105 which is not influenced by planets of the known solar system (other than Vulcan of course). And even that influence is slight.

We use Fourier analysis and related techniques to investigate the question of periodicities in fossil biodiversity. These techniques are able to identify cycles superimposed on the long-term trends of the Phanerozoic. We review prior results and analyze data previously reduced and published. Joint time-series analysis of various reductions of the Sepkoski Data, Paleobiology Database, and Fossil Record 2 indicate the same periodicity in biodiversity of marine animals at 62 Myr. We have not found this periodicity in the terrestrial fossil record. We have found that the signal strength decreases with time because of the accumulation of apparently “resistant” long-lived genera. The existence of a 62-Myr periodicity despite very different treatment of systematic error, particularly sampling-strength biases, in all three major databases strongly argues for its reality in the fossil record.

Note that peak exists at the nearly the same frequency and phase in three different data sets on fossil biodiversity.

We do not advocate any particular cause for this, but other papers discuss evidence for some of the possibilities.

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Episode 660: Crew Dragon Reaches the Station. What it Took to Replace the Space Shuttle

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